1. Field of the Invention
The present invention relates generally to set retarded cement compositions and methods of using the compositions for cementing subterranean zones in wells.
2. Description of the Prior Art
A variety of procedures involving hydraulic cement compositions are utilized in the completion and repair of wells such as oil and gas wells. For example, in the completion of a well after a well bore has been drilled into one or more subterranean producing formations, casing is typically disposed in the well bore and cemented therein. That is, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior of the casing. The cement composition is allowed to set in the annular space whereby an annular cement sheath is formed therein which bonds the casing to the walls of the well bore and prevents the undesirable flow of fluids into and through the annular space. This type of cementing procedure is referred to in the art as primary cementing.
In repairing producing wells, hydraulic cement compositions are often utilized to plug holes or cracks in the pipe disposed in the well bore; to plug holes, cracks, voids or channels in the above mentioned cement sheath between casing and the well bore; to plug permeable zones or fractures in subterranean formations and the like. The holes, cracks, etc. are repaired by forcing hydraulic cement compositions thereinto which harden and form impermeable plugs therein.
High temperatures are frequently encountered in deep subterranean zones to be cemented. The combination of the depth of the zone and the high temperature thereof often results in the requirement that the setting time of the cement composition be extended to allow the cement composition to be pumped into the zone to be cemented. This is particularly true in primary cementing whereby the cement composition must be pumped to the bottom of the well bore and then upwardly into the annulus between the casing and the well bore. The temperature at the bottom of the well bore through which the cement is pumped is typically the hottest location in the well bore.
A variety of set retarding additives have been developed and used for extending the time in which a cement composition sets. Such additives have been mixed with well cement compositions in amounts sufficient to delay the setting of the compositions until they can be pumped into desired subterranean locations.
While set retarding additives have been used successfully in well cement compositions heretofore, they can and often do produce erratic results in cement compositions of different component make-up. Also, a number of the prior set retarding additives cause premature gelation of cement compositions when the cement compositions are subjected to the high temperature environments of deep subterranean zones, i.e., temperatures of 200.degree. F. and higher. The gelation referred to is the abnormal increase in viscosity of a cement composition without an increase in compressive strength. The increase in viscosity makes the cement composition difficult or impossible to pump even though the cement composition has not set.
Thus, there is a continuing need for improved set retarded cement compositions and methods of using such compositions at high temperature without premature gelation and other problems being encountered.